Single and multiple deck conveyor dryers for reducing the moisture content of sheet materials, including green (i.e. wet) wood veneer, wherein the material being dried is conveyed through a stationary drying chamber while heated gases are circulated through the drying chamber, are well-known in the art. Evaporation of moisture from the material being dried causes a build-up of steam within the dryer and creates a positive pressure differential within the dryer relative to the external atmosphere. Disposal of large amounts of gases containing volatile organic compounds (V.O.C.) which are evolved from the wood veneer during drying has been a major problem in the wood veneer drying art. Typically, gases are removed by an exhaust system and, in some systems, are exhausted directly to the atmosphere. By allowing gases to be exhausted directly to the atmosphere, pollutants have been allowed to escape and considerable quantities of heat energy are lost which is a considerable cost expense.
It is desirable to control the exhaust of gases from a wood veneer dryer to optimize the drying efficiency of the dryer and to provide a means for containing and treating the exhaust gases prior to discharge into the atmosphere. Such means include installing a V.O.C. separating device such as a catalytic or thermal oxidizer in the exhaust system. Such devices are well-known in the art.
If the drying process is not carefully controlled and optimized, untreated gases (i.e. gases containing V.O.C.) within a dryer will be discharged through not only the exhaust system, but through input and output ends of the dryer due to the positive pressure differential within the dryer. This presents real environmental concerns and health and safety hazards to workers. Prior art dryers have other disadvantages, including a loss of efficiency due to the entrance of air at ambient temperature into the dryer through input and output ends if a negative pressure differential is created within the dryer relative to the external atmosphere if gases are exhausted too quickly. Entrance of cooler ambient air reduces the temperature of the dryer, wasting a considerable amount of energy to reheat the dryer and resulting in pitch (i.e. condensed V.O.C. material) build-up within the dryer. This is an obvious fire hazard. Attempts have been made to control the inflow and outflow of gases through the input and output ends of wood veneer dryers using stop-offs or baffles. An example of one such attempt to improve wood veneer dryer efficiency and safety is disclosed in U.S. Pat. No. 4,439,930. However, with use, the stop-offs or baffles wear and the inflow and outflow of gases from the input and output ends of the dryer increases. To replace or adjust the worn stop-offs or baffles, the operator is required to stop operation of the wood veneer dryer to gain access to the stop-offs or baffles inside the dryer once the dryer has cooled. To access the worn stop-offs or baffles, one or more of dryer rollers, jet tubes, and conveyor chains of the dryer must be removed. Typically, about three operators are required to then remove the heavy stop-off or baffle. The process to replace or adjust worn stop-offs or baffles typically takes approximately 16 to 20 hours. The time the dryer spends in repair and/or is otherwise nonoperational, the required manpower, and the energy required to return a dryer to wood veneer drying temperatures, present significant costs.
There is accordingly a need in the art for a wood veneer dryer wherein the inflow of cooler ambient air into the dryer and/or the outflow of gases from inside the dryer are controlled to reduce energy consumption by the dryer, prevent the build-up of pitch within the dryer, and/or prevent the outflow of gases containing V.O.C. from the dryer before treatment in the exhaust system.
The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.